Your SR goes down, but not any massive amount, it more than evens out if you're a decent driver. In Rookie there are obviously more crashes, because it's rookie, and the goal is to not give a **** about results and simply concentrate on SR, as the standard of driving improves dramatically after you get out.
After that, obviously people crash, but as the races get longer it gets harder and harder to lose SR. You can do a 30 lap F1 race at Road America, getting 12 incs (3 seperate crashes) and still gain SR.
It's nowhere near as bad as some make out. Of course, it's a no fault system which has it's drawbacks, but it's not really a problem. Now I'm Class A I don't even think about SR, I don't crash obviously because I would be out of the race, but if I drop a couple of tyres off and get a few 1x penalties while I'm chasing someone down, then that's fine and more than worth it.